Marine animal monitoring has been performed to study populations of fish and marine mammals for academic research and commercial purposes. In addition, monitoring may be mandated for certain operations. For example, the Marine Mammal Protection Act (MMPA) of 1972, Amended 1994 requires marine mammal monitoring for operations that involve the transmission of acoustic or seismic signals, such as occurs in marine seismic and geophysical surveys and in sonar surveillance. Active detection methods generally consist of sending underwater signals and capturing the reflections of these signals from objects, such as marine animals. However, active detection methods are not preferred because the transmitted underwater signals may affect the behavior of the marine animals being surveyed and may have detrimental effects on marine animals and their environment. Passive detection systems involve monitoring visible and audible information of marine mammals without introducing signals into the environment.
Conventional passive monitoring systems operate using a single hydrophone or sparse hydrophone (for instance, two to four hydrophones) system elements to detect underwater acoustic signals and by positioning a group of observers (for instance, about four to six observers) using binoculars to visually detect marine mammals. The detection areas of such hydrophone systems are generally limited to about less than 100 square kilometers (km2) in the immediate vicinity of a vessel towing the hydrophone system. Conventional monitoring systems are time consuming, experience frequent costly disruptions, and only provide small areal coverage relative to the bodies of water being surveyed. In addition, conventional monitoring systems are not reliable for monitoring many types of marine mammals, such as baleen whales. Accordingly, marine mammal monitoring operations would benefit from passive monitoring systems and techniques that are capable of efficiently and effectively monitoring larger areas while minimizing loss of survey time.